I also had that in one family.
Basically, as soon as I clicked adopt on one of the previous generation kids, Dollar, the adoption event happened! So basically, Dollar and daughter, Etna, both walked into the house for the first time together. It was definitely a cute happenstance.
I also used it as a way to try and hold out for a perfect match. In fact, when Etna was about 9, Dollar was still unmarried, but there was another adoption event and ended up with a second daughter Lucette. I eventually realized the chances of a perfect match were slim and at 48 finally let her enjoy marriage with a nice fellow named Marcolo. They immediately had a son named Boi.